Electronic printing where there is direct charge transfer onto the surface of a substrate has necessarily in the past been done by depositing charge on a dielectric substrate which has a fixed or deposited conductive ground plane or backing plate to the dielectric substrate. In the case of display technology which require back lighting where it is desirable to have a transparent substrate such a deposited conductive ground plane has had to be of a transparent nature. For instance with colour liquid crystal display screens for lap top computers which are backlit and hence cannot have an opaque conductor on the rear face it is desirable to have some other method of laying down the colour filter matrix on the substrate. There are many applications, however, where it is not cost effective, convenient or possible to have a conductive ground plane on this substrate for subsequent processing steps and final use of the product.
The prior art discloses several methods of laying down a charge pattern onto a dielectric. U.S. Pat. No. 5,124,730 assigned to Armstrong World Industries Inc discloses laying down a thin removable dielectric surface onto a conductive belt, laying down a charge pattern onto the conductive belt depositing and fixing a toner onto the dielectric layer and then removing the dielectric layer. There is no requirement in this patent that the dielectric layer be transparent as the invention is particularly adapted for printing onto thin vinyl sheets which are subsequently placed onto backings for vinyl floor tiles and wallpapers. The dielectric layer is extremely thin, being in the order of from 0.2 to 10 mils (5 to 250 microns) thick as it appears that thicker layers will not hold a charge during the laying down of the charge pattern and depositing the toner. Japanese Patent specification Publication No. 05-127015 in the name of Toyo Ink MFG Co Ltd uses electronic printing to form an optical colour filter matrix to be used in a colour television or liquid crystal display. To enable deposition of the optical colour filter there is formed a photoconductive layer onto the substrate and the image is formed onto the photoconductive layer by masking and light. This disclosure does not show direct laying down of a charge pattern onto the substrate but uses photoconductive and masking technology to lay down the charge pattern.
Japanese Patent specification Publication No. 03-150503 in the name of Toppan Printing Co Ltd uses a mask to define the charge pattern on the surface of a substrate with a conductor affixed to the rear surface of the substrate the conductor disclosed is an indium tin oxide transparent conductor. As discussed above this is unacceptable for some applications.
The problem to be addressed by the present invention is that where there is not an integral ground plane on the substrate decay of the charge pattern laid down on the surface of the substrate can easily occur and the electrostatic latent image is easily perturbed by the presence of a charged toner during the toner development step. This can lead to loss of resolution and/or displacement of toner particles. Particularly when three colour printing is done this problem is of serious concern.
It is the object of this invention to provide a method of formation and development of an electrostatic image on a dielectric substrate which does not intrinsically distort, discharge or pertubate the electric field emanating from the dielectric substrate.
The present invention has particularly application within the display industry and in particular the electronic display industry for instance with respect to liquid crystal displays. In one embodiment the invention allows direct imaging on so-called black matrix glass such that the colour filters can be directly formed on such glass by way of electrostatic attraction of the pigments and associated binder resins without the need to coat the glass with a conductive layer on the opposite side of the glass.
The invention is not limited to this application, however, but is applicable to any system where toner is to be laid down onto a dielectric substrate which does not or cannot include a conductive ground plane.